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EVER since the Apollo 11 flight to the Moon in 1969, astronauts have reported seeing weird spots, streaks and clouds of light. Now we know the cause.

¿ìè¶ÌÊÓƵs had already worked out that the most likely cause was high-energy particles from space, known as cosmic rays, hitting the astronauts’ eye. But they didn’t know which particles were involved, how they cause the flashes, and whether there is any risk to health.

So between 1995 and 1999, 10 astronauts on the Russian space station Mir took part in tests to try to work out what was happening. They wore a helmet with a particle detector mounted over one eye that measured the charge, mass and direction of cosmic rays passing through it, and were told to push a button on a joystick whenever they witnessed a flash of light.

Although about 90 per cent of cosmic rays are protons, the results, published this month, show that the astronauts were most likely to experience light flashes when much rarer kinds of cosmic ray hit their eyes. A heavier high-energy ion such as helium and lithium nuclei is a thousand times more likely to cause a flash than an incoming proton.

It had been thought that cosmic rays might be creating Cerenkov radiation, the electromagnetic equivalent of a sonic boom, within the vitreous humour of the eye. But that would be just as readily produced by protons as heavy ions. So it seems instead that the cosmic ray are depositing energy directly in the retina. Exactly how this energy turns into a perceived light flash is unclear, however, making it hard to assess the health risk. This week a Russian Soyuz rocket was due to carry equipment to the International Space Station for more tests.